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antibodies: molecules (also called immunoglobulins) produced by a B cell in response to an antigen. When an antibody attaches to an antigen, it destroys the antigen.

antigen: a substance or molecule that is recognized by the immune system. The molecule can be from a foreign material such as bacteria or viruses.

B cells: small white blood cells crucial to the immune defenses. Also known as B lymphocytes, they come from bone marrow and develop into blood cells called plasma cells, which are the source of antibodies.

bacteria: microscopic organisms composed of a single cell and lacking a defined nucleus and membrane-enclosed internal compartment. carriers — apparently healthy people who harbor disease-causing microbes in the body and who can infect others by passing the microbes on to them.

cell: the smallest unit of life; the basic living unit that makes up tissues.

disease: a state in which a function or part of the body is no longer in a healthy condition.

DNA (deoxyribonucleic acid): a complex molecule found in the cell nucleus which contains an organism's genetic information. epidemic — a disease outbreak that affects many people in a region at the same time.

genes: units of genetic material (DNA) that carry the directions a cell uses to perform a specific function.

human immunodeficiency virus (HIV): the virus that causes AIDS.

immune system: a complex network of specialized cells, tissues, and organs that defends the body against attacks by disease-causing microbes.

immunization: vaccination or other process that induces protection (immunity) against infection or disease caused by a microbe.

infection: a state in which disease-causing microbes have invaded or multiplied in body tissues.

infectious diseases: diseases caused by microbes that can be passed to or among humans by several methods.

inflammation: an immune system process that stops the progression of disease-causing microbes, often seen at the site of an injury like a cut. Signs include redness, swelling, pain, and heat. latent — present but not seen. A latent viral infection is one in which no virus can be found in the blood cells but in which those virus-infected cells can produce virus under certain circumstances. meningitis — inflammation of the meninges, the membranes that surround the brain and spinal cord.

molecules: the smallest physical units of a substance that still retain the chemical properties of that chemical substance; molecules are the building blocks of a cell. Some examples are proteins, fats, carbohydrates, and nucleic acids.

organisms: individual living things.

pandemics: diseases that affect many people in different regions around the world.

parasites: plants or animals that live, grow, and feed on or within another living organism.

pathogens: disease-causing organisms.

RNA (ribonucleic acid): a complex molecule that is found in the cell cytoplasm and nucleus. One function of RNA is to direct the building of proteins.

rotavirus: a group of viruses that can cause digestive problems and diarrhea in young children.

T cells: small white blood cells (also known as T lymphocytes) that direct or directly participate in immune defenses. tissues: groups of similar cells joined to perform the same function.

toxins: agents produced by plants and bacteria, normally very damaging to human cells.

vaccines: substances that contain parts of antigens from an infectious organism. By stimulating an immune response (but not disease), they protect the body against subsequent infection by that organism.

Source: National Institute of Allergy and Infectious Diseases (NIAID). NIAID is a component of the National Institutes of Health (NIH), which is an agency of the Department of Health and Human Services. NIAID supports basic and applied research to prevent, diagnose, and treat infectious and immune-mediated illnesses. It does not endorse any of the products or services advertised on this Web site. Common Cold Fact Sheet last updated December 2007. Flu Fact Sheet last updated September 2008.

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